As you mention Datastores I guess you are using VMware and not Hyper-V

With regards the snaphot process - Backup Exec requests that VMware snapshots the whole VM. We in effect do the same thing that happens if you use the VMware vSpeher Client Snapshot Manager to snapshot a whole VM (with quiesce enabled).

VMware itself is then responsonsible for managing how the snapshots are created including when vmdk files owned by the VM are held in different datatstores.

Note if you have database inside the VM (such as Exchange or MSSQL) then VMware tools issues a VSS request inside the VM guest operating system to make sure that the database data is consistent.

Also if multiple VMs are in one backup job, each VM is snapped as we reach that VM in order (they are not all snapshotted at the start of the job)

A snapshot will be created on the same datastore as the disk is located. In your case on 3 different datastores.

The additional required space depends on how much data is changed on the disks during the snapshot, as all changes are written to the snapshot file. When the snapshot is removed these changes are commited in the original disks.
If there are not much changes, the extra required space is very limited.